Personal behavior is the only factor that determines if a person becomes ill true or false

False.

There are many perspectives, factors and elements that may influence or initiate a person's sickness or dysfunction. 
As many researches would suggest there are many other factors to consider and shed light into this phenomenon. 

For example, obesity which is a going rampant and rapid eating and weight disorder.
In a biopsychological perspective, there may causes of one, the lipolysis function of the body isn't as rapid or the same with most individuals since the findings might suggest that some individuals have a lower metabolic rate or enzyme to process the lipids of a person.
In a psychological perspective, the patient might be suffering from a severe self-conflict which leads to rapid food intake or depressive state that might have caused these overweight problems.
In another heredity, obesity might be genetic.

Target heart rate is the answer